High-purity silica has been increasingly used in the recent years for manufacture of electronic materials and semiconductors as electronic industries have rapidly developed. In proportion with development of high-performance articles, not only higher purity but also improvement of other performances such as restriction of particle top size, high fluidity at the time of resin blending, etc. have become intensely required. Particularly, in the field of flip chip type semiconductor devices, non-porous spherical silica low in the coarse particle content and excellent in fluidity at the time of resin blending has become required as an underfill filler. In the field of underfill filler where a particular importance is on the viscosity at the time of resin blending, silica having a low viscosity and a low thixotropic ratio has become needed. As used herein, the term “thixotropic ratio” generally means a value obtained by measuring viscosity at varied rotation speed by a rotational viscometer such as B type viscometer and dividing the viscosity at a low rotation speed by the viscosity at a high rotation speed. Among the resins, epoxy resins and silicone resins are useful, and non-porous spherical silica showing a low thixotropic ratio particularly when mixed with Bisphenol A type liquid epoxy resin has been required.
As non-porous spherical silica low in coarse particle content, excellent in fluidity at the time of resin blending and showing a low thixotropic ratio,    1) fused spherical silica characterized in that the maximum particle diameter is 45 μm, the mean particle diameter is 2–10 μm, the ratio Sw1/Sw2 is 1.0–2.5 (Sw1 is specific surface area of the particle and Sw2 is theoretical specific surface area of the particle), and surface of the particle is smooth (JP-2000-7319),    2) fine spherical silica characterized in that the maximum particle diameter is 24 μm, the mean particle diameter is 1.7–7 μm, it has a particle size distribution with the value X1 of not smaller than 100/D50% by weight and not greater than (18+100/D50) by weight (X1 represents proportion of the particles having a size not larger than 3 μm to the total particles), and a mixture obtained by blending the fine spherical silica into liquid epoxy resin or silicone resin at ordinary temperature with a blending ratio of at most 80% by weight has a viscosity at 50° C. of 20 Pa·s or less (JP-2000-63630),etc. have so far been proposed.
However, since the non-porous spherical silica 1) is obtained by using fused spherical silica as a starting material, even if the superficial fine powder is dissolved away, the surface of the particle itself is also dissolved to leave irregularities on the particle surface and the superficial silanol residues. An expected fluidity cannot be obtained at the time of resin blending for this reason. The non-porous spherical silica 2) is also produced by substantially using fused spherical silica and, thus, has fine powder on its surface. Even if the fine powder is dissolved away as in the production of 1), an expected fluidity cannot be obtained for the same reason as mentioned above. Although the non-porous spherical silica of 1) and 2) show a low thixotropic ratio when measured at 5–10 rpm with viscometer, the viscosity tends to rise at a low rotation speed (i.e. thixotropic ratio tends to become greater).
This is not desirable for an application, such as an underfill material, in which silica is made to permeate by a mere capillary phenomenon in a static state. Thus, a filler having even lower thixotropic ratio has been awaited.
On the other hand, as processes for producing non-porous spherical silica, the followings have so far been proposed:    1) a process comprising melting crushed high-purity silica in flame and dissolving away the fine powder on the surface with an alkali or hydrofluoric acid (e.g. JP-2000-7319),    2) a process comprising hydrolyzing silicon alkoxide to obtain a sol-like solution and making the particles grow up in the sol (e.g. JP-B-2-288),    3) a process comprising hydrolyzing silicon alkoxide to obtain a sol-like solution, making the particles grow up to obtain spherical silica gel, and calcinating the spherical silica gel with flame (e.g. JP-A-2-296711),    4) a process for producing high-purity spherical silica, characterized by mixing a water-in-oil type (W/O type) emulsion prepared by finely dispersing an aqueous solution of alkali silicate as a dispersed phase with an water-in-oil type (W/O type) emulsion prepared by finely dispersing an aqueous solution of a mineral acid as a dispersed phase to form spherical silica gel, treating the spherical silica gel with a mineral acid to obtain spherical hydrated silica, drying the hydrated silica, followed by calcinating (JP-A-07-069617), and    5) a process comprising mixing fused spherical silica having a mean particle diameter of 2–7 μm from which coarse particles have been removed by sieving with spherical silica having a mean particle diameter of 0.3–1.0 μm which has been produced from metallic silicon (JP-A-2000-63630).
All the above-mentioned conventional processes for producing spherical silica respectively have disadvantages as mentioned below.
Thus, in the particle obtained by process 1), when the fine powder on its surface is dissolved away with an alkali or hydrofluoric acid, surface of the objective particle itself is also dissolved and, thus, the surface of the resultant particle is in a roughened state. Consequently, its fluidity at the time of resin blending cannot be high, even though the specific surface area is close to the theoretical value. Further, the dissolution of objective particle occurring simultaneously with the dissolution of fine powder on the surface reduces the product yield, to adversely affect its productivity.
In the process 2), sol particles are formed and grown by a hydrolytic polycondensation reaction of silicon alkoxide. Accordingly, the resulting product is spherical silica gel formed from relatively large primary particles. Traces of the large primary particles remain even after sintering the particles by heat-treatment to make them non-porous. Thus, the fluidity cannot be high even though the specific surface area is close to theoretical value.
In the process 3), silicon alkoxide is hydrolyzed to obtain a sol solution, the particles are grown, and the spherical silica gel thus obtained is made non-porous in a flame. However, since the temperature at which the treatment is carried out is 1,500° C., which is lower than the melting point, only the spherical silica that is comparable to that obtained by the process 2) is obtained.
Further, the spherical silica particles obtained by the processes 2) and 3) largely contract upon calcinating to form irregularities on the surface and the resultant particles are inferior in superficial smoothness. A decrease in superficial smoothness is undesirable because it causes a decrease in fluidity. Further, the processes 2) and 3) use expensive starting materials and generate waste-water containing organic materials derived from the starting materials and thus, necessitate treatment of the waste-water.
The process 4) gives spherical silica particles excellent in both sphericity and fluidity. However, upon calcinating, a part of the particles coagulate therebetween. The particles, were, therefore, found to contain some particles with particle size exceeding 4 times the mean particle diameter, similarly to the products of processes 1), 2) and 3).
In the process 5), since fused spherical silica is blended, fine powder on the particle surface disturb the fluidity at the time of resin blending, resulting in unsatisfactory flowing property. Further, in all the above-mentioned prior art processes, mechanical classification has to be carried out in order to attain a particle size distribution with a low coarse particle content.
Although such mechanical classification methods include a method using a screen, a gas stream classification method, etc., it is quite difficult to completely classify the particles having a mean particle diameter of about 0.1–20 μm to such an extent that the maximum particle diameter reaches a value 4 times the mean particle diameter or less.
In other words, it has hitherto been impossible to obtain non-porous spherical silica having the maximum particle diameter of 4 times the mean particle diameter or less, as well as a low thixotropic ratio.
It is an object of the present invention to provide non-porous spherical silica which hardly lowers fluidity of a liquid sealing material when it is blended therewith at the filling rate based on the liquid sealing material in the range of 40–90% by weight, which does not block a gap when used as a filler in a liquid sealing material to be poured into narrow gaps, having such a particle size distribution that the maximum particle diameter is 4 times the mean particle diameter or less, and showing a low thixotropic ratio upon being blended with resin; and to provide an industrially advantageous process for production of the same.